In the Thumbnail Preview (Shift+T in Panels), when an image file is
removed, Nomacs normally notices this and removes the corresponding
thumbnail. But when multiple files are removed at once (e.g. from a
shell by using the wildcard *), Nomacs removes some of them, but not
all of them. If I double-click on such a thumbnail that hasn't been
removed, then Nomacs can't display it (obviously) and displays the
first image (among all shown thumbnails) instead; then Shift-T puts
the slider at the top (corresponding to the first image) instead of
the last position.